Thanks VB and MB. Before I pass the amp on to someone that knows what they are doing I was wondering if you feel it would be worthwhile me ordering a new set of valves for the output stage to see if swapping them for the existing ones will solve the problem. If I put the new valves in and it wasnt faulty valves causing these problems, would the new valves be at risk?

Personally I think buying new valves would be quite a big investment with a relatively limited chance of success. Capacitors fail catastrophically more often than valves do. Do you know anyone with a valve tester who could check your existing ones over for you (sorry, despite the name I don't have one, not yet at least ...) ? A request on here might get a response. If you did buy a new set then it's unlikely (but not quite impossible) that you'd damage them in a quick test. I wouldn't run them for more than a few seconds though. And I'd probably substitute, say, just one pair with new ones rather than both pairs. Or maybe even just one valve ? And at least you'd have a spare set, which is always a comforting feeling. Prices are not going down.

Thanks very much Paul and VB(again) for help and advice. Before I read your replied I had some time this PM to play around with some old(sacrificial) speakers to see if I could pinpoint the problem. Seems to me that the problem is a faulty valve as the hideous noise being emitted from the speaker seemed to follow this particular valve as it travelled from channel to channel and back again.

Not wanting to experience this problem, seems likely that the other EL84s would be of a similar age, I am just off to the hifi collective to buy me some nice new quad matched EL84s. I think/hope/pray that will be the end of this nightmare.

Thanks to everyone for their advice and help with this. I will report back when the new valves are in place.

Ordered a quad matched set of Sovtek EL84M/6BQ5 Military Spec from HotRoxUK yesterday. Selected budget delivery option but it appears they have been delivered today. Cant wait to get home, put them in and make sure that they solve this problem I have been having. Fingers crossed!

Well thats fucked it! Got home all excited about getting my amp working again. Put all the new valves in and....

Nothings fucking changed!!!!

Same very worrying noise being emmitted from the speakers as the valves start to warm up....

So what is the likelihood that it is one of the 3 remaining ECC83 valves that i havent changed yet. They are all Full Music 12AX7's and look bloody expensive to replace like for like so maybe I should get some cheap ones to see if they are the problem? Or maybe I should just take it to a reputable service tech - anyone know anyone in the West Midlands?

I don't think we have enough information to guess what the fault is I'm afraid. When you were looking at the output valves earlier you thought that the fault had followed a valve from one channel to the other. If you can be sure about that, i.e. that both channels have, at one time or another, exhibited this fault then it has to be something which is common to both channels (say the power supply ?) or it has to be something which you moved from one channel to the other (one or more EL84s, assuming you changed nothing else). It's now looking like the root cause is not the EL84s though. I guess the ECC83s are not common to both channels either so unless the same fault has appeared simultaneously in two of them then the root cause will be somewhere else. The '83s may or may not be damaged, as Robin's were, but that's not the root cause and it's that that you have to track down. It sounds like it's time to get a tech involved. He'll have access to the kit needed to isolate the fault which will be tough, and potentially expensive in unnecessary spares, if you try to do it from scratch.

already been in touch with Jack as I bought the amp from him. He is very busy with his Ming Da stuff at the mo and life in general so dont want to bother him too much. I have used good ol google and found a company/tech that looks very good and is very local:

Not disputing that Chris but why have a "pre-amp out" output if that is the case? I used the Oto as a Pre to my active speakers for several hours and it didnt seem to do it any harm(at the time that is...). Do other valve integrateds have pre-outs and if so what are you supposed to do if you want to use that function???